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From: Anthony Williams (anthony_at_[hidden])
Date: 2008-04-21 03:26:15

Quoting Patrick Twohig <p-twohig_at_[hidden]>:

> I've read doug Schmidt's paper a little while back when I was trying to
> implemented a condition variable on a system lacking support for it.
> Additionally, SignalObjectAndWait was not available because the code used
> CriticalSections instead of Windows Mutexes so I ended up basing the
> implementation on some code I found which used a deque of signals to notify
> one or all threads. I don't know if that approach introduces fairness
> problems or not, but it seemed to work for me. I'm certainly not suggesting
> tearing apart all the code in boost::condition_variable with something
> similar, but I was just more comparing boost's implementation with with a
> different implementation I had found.

There's lots of ways of implementing condition variables. One of my
earlier prototypes was like that. It's essentially a LIFO or FIFO
scheduler. The issue here is that the OS scheduler doesn't control the
wake order, so high priority threads might lose out to low priority

If I can't fix the current problems in a sensible manner, I might
revert to that prototype, or I might revert to the old
boost::condition algorithm. The reason I changed from the old
algorithm is that a notify has to wait until the notified thread has
woken before it can resume. This might result in a high-priority
thread waiting on a low priority thread.


Anthony Williams            | Just Software Solutions Ltd
Custom Software Development |
Registered in England, Company Number 5478976.
Registered Office: 15 Carrallack Mews, St Just, Cornwall, TR19 7UL

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